Installed Canon MP630 drivers from .deb files on Ubuntu, and then copied the
/usr/share/ppd directory, the /etc/cups/cupsd.conf and printers.conf, and the
/etc/cups/ppd/MP630-series.ppd files across to Gentoo (some of these files
appear to be binary drivers). Printing from Gentoo then worked.

Note that USB printers have to be attached and powered up before the system
is booted, at least for the install.

There are two extra directories in Ubuntu, /usr/share/cngpijmonmp630 and
/usr/share/printuimp630; they seem to be part of cnijtools, but were copied
across anyway.